Hydrocarbon fuels generally contain numerous deposit-forming substances. When used in internal combustion engines, deposits tend to form on and around constricted areas of the engine in contact with the fuels. In diesel engines, deposits tend to accumulate in the fuel injection system, thereby hampering good performance of the engine. In spark ignition engines deposits can build up on engine intake valves leading to progressive restriction of gaseous fuel mixture flow into the combustion chamber and also to valve sticking. It is common practice therefore to incorporate a detergent in the fuel composition for the purpose of inhibiting the formation, and facilitating the removal, of engine deposits, thereby improving engine performance. Mannich condensation products, obtained by reacting hydrocarbon-substituted phenols with aldehydes and amines, are known as detergents for fuels.